#include "recovery.h"
 #include "replicas.h"
 #include "sb-clean.h"
+#include "sb-downgrade.h"
 #include "snapshot.h"
 #include "subvolume.h"
 #include "super-io.h"
                        printbuf_exit(&buf);
                }
 
+               if (bch2_check_version_downgrade(c)) {
+                       struct printbuf buf = PRINTBUF;
+
+                       prt_str(&buf, "Version downgrade required:\n");
+
+                       __le64 passes = ext->recovery_passes_required[0];
+                       bch2_sb_set_downgrade(c,
+                                       BCH_VERSION_MINOR(bcachefs_metadata_version_current),
+                                       BCH_VERSION_MINOR(c->sb.version));
+                       passes = ext->recovery_passes_required[0] & ~passes;
+                       if (passes) {
+                               prt_str(&buf, "  running recovery passes: ");
+                               prt_bitflags(&buf, bch2_recovery_passes,
+                                            bch2_recovery_passes_from_stable(le64_to_cpu(passes)));
+                       }
+
+                       bch_info(c, "%s", buf.buf);
+                       printbuf_exit(&buf);
+                       write_sb = true;
+               }
+
                if (check_version_upgrade(c))
                        write_sb = true;
 
        c->disk_sb.sb->compat[0] |= cpu_to_le64(1ULL << BCH_COMPAT_extents_above_btree_updates_done);
        c->disk_sb.sb->compat[0] |= cpu_to_le64(1ULL << BCH_COMPAT_bformat_overflow_done);
 
-       bch2_sb_maybe_downgrade(c);
+       bch2_check_version_downgrade(c);
 
        if (c->opts.version_upgrade != BCH_VERSION_UPGRADE_none) {
                bch2_sb_upgrade(c, bcachefs_metadata_version_current);
 
--- /dev/null
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Superblock section that contains a list of recovery passes to run when
+ * downgrading past a given version
+ */
+
+#include "bcachefs.h"
+#include "darray.h"
+#include "recovery.h"
+#include "sb-downgrade.h"
+#include "sb-errors.h"
+#include "super-io.h"
+
+/*
+ * Downgrade table:
+ * When dowgrading past certain versions, we need to run certain recovery passes
+ * and fix certain errors:
+ *
+ * x(version, recovery_passes, errors...)
+ */
+
+#define DOWNGRADE_TABLE()
+
+struct downgrade_entry {
+       u64             recovery_passes;
+       u16             version;
+       u16             nr_errors;
+       const u16       *errors;
+};
+
+#define x(ver, passes, ...) static const u16 ver_##errors[] = { __VA_ARGS__ };
+DOWNGRADE_TABLE()
+#undef x
+
+static const struct downgrade_entry downgrade_table[] = {
+#define x(ver, passes, ...) {                                  \
+       .recovery_passes        = passes,                       \
+       .version                = bcachefs_metadata_version_##ver,\
+       .nr_errors              = ARRAY_SIZE(ver_##errors),     \
+       .errors                 = ver_##errors,                 \
+},
+DOWNGRADE_TABLE()
+#undef x
+};
+
+static inline const struct bch_sb_field_downgrade_entry *
+downgrade_entry_next_c(const struct bch_sb_field_downgrade_entry *e)
+{
+       return (void *) &e->errors[le16_to_cpu(e->nr_errors)];
+}
+
+#define for_each_downgrade_entry(_d, _i)                                               \
+       for (const struct bch_sb_field_downgrade_entry *_i = (_d)->entries;             \
+            (void *) _i        < vstruct_end(&(_d)->field) &&                          \
+            (void *) &_i->errors[0] < vstruct_end(&(_d)->field);                       \
+            _i = downgrade_entry_next_c(_i))
+
+static int bch2_sb_downgrade_validate(struct bch_sb *sb, struct bch_sb_field *f,
+                                     struct printbuf *err)
+{
+       struct bch_sb_field_downgrade *e = field_to_type(f, downgrade);
+
+       for_each_downgrade_entry(e, i) {
+               if (BCH_VERSION_MAJOR(le16_to_cpu(i->version)) !=
+                   BCH_VERSION_MAJOR(le16_to_cpu(sb->version))) {
+                       prt_printf(err, "downgrade entry with mismatched major version (%u != %u)",
+                                  BCH_VERSION_MAJOR(le16_to_cpu(i->version)),
+                                  BCH_VERSION_MAJOR(le16_to_cpu(sb->version)));
+                       return -BCH_ERR_invalid_sb_downgrade;
+               }
+       }
+
+       return 0;
+}
+
+static void bch2_sb_downgrade_to_text(struct printbuf *out, struct bch_sb *sb,
+                                     struct bch_sb_field *f)
+{
+       struct bch_sb_field_downgrade *e = field_to_type(f, downgrade);
+
+       if (out->nr_tabstops <= 1)
+               printbuf_tabstop_push(out, 16);
+
+       for_each_downgrade_entry(e, i) {
+               prt_str(out, "version:");
+               prt_tab(out);
+               bch2_version_to_text(out, le16_to_cpu(i->version));
+               prt_newline(out);
+
+               prt_str(out, "recovery passes:");
+               prt_tab(out);
+               prt_bitflags(out, bch2_recovery_passes,
+                            bch2_recovery_passes_from_stable(le64_to_cpu(i->recovery_passes[0])));
+               prt_newline(out);
+
+               prt_str(out, "errors:");
+               prt_tab(out);
+               bool first = true;
+               for (unsigned j = 0; j < le16_to_cpu(i->nr_errors); j++) {
+                       if (!first)
+                               prt_char(out, ',');
+                       first = false;
+                       unsigned e = le16_to_cpu(i->errors[j]);
+                       prt_str(out, e < BCH_SB_ERR_MAX ? bch2_sb_error_strs[e] : "(unknown)");
+               }
+               prt_newline(out);
+       }
+}
+
+const struct bch_sb_field_ops bch_sb_field_ops_downgrade = {
+       .validate       = bch2_sb_downgrade_validate,
+       .to_text        = bch2_sb_downgrade_to_text,
+};
+
+int bch2_sb_downgrade_update(struct bch_fs *c)
+{
+       darray_char table = {};
+       int ret = 0;
+
+       for (const struct downgrade_entry *src = downgrade_table;
+            src < downgrade_table + ARRAY_SIZE(downgrade_table);
+            src++) {
+               if (BCH_VERSION_MAJOR(src->version) != BCH_VERSION_MAJOR(le16_to_cpu(c->disk_sb.sb->version)))
+                       continue;
+
+               struct bch_sb_field_downgrade_entry *dst;
+               unsigned bytes = sizeof(*dst) + sizeof(dst->errors[0]) * src->nr_errors;
+
+               ret = darray_make_room(&table, bytes);
+               if (ret)
+                       goto out;
+
+               dst = (void *) &darray_top(table);
+               dst->version = cpu_to_le16(src->version);
+               dst->recovery_passes[0] = cpu_to_le64(src->recovery_passes);
+               dst->recovery_passes[1] = 0;
+               dst->nr_errors          = cpu_to_le16(src->nr_errors);
+               for (unsigned i = 0; i < src->nr_errors; i++)
+                       dst->errors[i] = cpu_to_le16(src->errors[i]);
+
+               table.nr += bytes;
+       }
+
+       struct bch_sb_field_downgrade *d = bch2_sb_field_get(c->disk_sb.sb, downgrade);
+
+       unsigned sb_u64s = DIV_ROUND_UP(sizeof(*d) + table.nr, sizeof(u64));
+
+       if (d && le32_to_cpu(d->field.u64s) > sb_u64s)
+               goto out;
+
+       d = bch2_sb_field_resize(&c->disk_sb, downgrade, sb_u64s);
+       if (!d) {
+               ret = -BCH_ERR_ENOSPC_sb_downgrade;
+               goto out;
+       }
+
+       memcpy(d->entries, table.data, table.nr);
+       memset_u64s_tail(d->entries, 0, table.nr);
+out:
+       darray_exit(&table);
+       return ret;
+}
+
+void bch2_sb_set_downgrade(struct bch_fs *c, unsigned new_minor, unsigned old_minor)
+{
+       struct bch_sb_field_downgrade *d = bch2_sb_field_get(c->disk_sb.sb, downgrade);
+       if (!d)
+               return;
+
+       struct bch_sb_field_ext *ext = bch2_sb_field_get(c->disk_sb.sb, ext);
+
+       for_each_downgrade_entry(d, i) {
+               unsigned minor = BCH_VERSION_MINOR(le16_to_cpu(i->version));
+               if (new_minor < minor && minor <= old_minor) {
+                       ext->recovery_passes_required[0] |= i->recovery_passes[0];
+                       ext->recovery_passes_required[1] |= i->recovery_passes[1];
+
+                       for (unsigned j = 0; j < le16_to_cpu(i->nr_errors); j++) {
+                               unsigned e = le16_to_cpu(i->errors[j]);
+                               if (e < BCH_SB_ERR_MAX)
+                                       __set_bit(e, c->sb.errors_silent);
+                               if (e < sizeof(ext->errors_silent) * 8)
+                                       ext->errors_silent[e / 64] |= cpu_to_le64(BIT_ULL(e % 64));
+                       }
+               }
+       }
+}
 
 #include "replicas.h"
 #include "quota.h"
 #include "sb-clean.h"
+#include "sb-downgrade.h"
 #include "sb-errors.h"
 #include "sb-members.h"
 #include "super-io.h"
        bch2_sb_members_from_cpu(c);
        bch2_sb_members_cpy_v2_v1(&c->disk_sb);
        bch2_sb_errors_from_cpu(c);
+       bch2_sb_downgrade_update(c);
 
        for_each_online_member(ca, c, i)
                bch2_sb_from_fs(c, ca);
 }
 
 /* Downgrade if superblock is at a higher version than currently supported: */
-void bch2_sb_maybe_downgrade(struct bch_fs *c)
+bool bch2_check_version_downgrade(struct bch_fs *c)
 {
+       bool ret = bcachefs_metadata_version_current < c->sb.version;
+
        lockdep_assert_held(&c->sb_lock);
 
        /*
        if (c->sb.version_min > bcachefs_metadata_version_current)
                c->disk_sb.sb->version_min = cpu_to_le16(bcachefs_metadata_version_current);
        c->disk_sb.sb->compat[0] &= cpu_to_le64((1ULL << BCH_COMPAT_NR) - 1);
+       return ret;
 }
 
 void bch2_sb_upgrade(struct bch_fs *c, unsigned new_version)
 {
        lockdep_assert_held(&c->sb_lock);
 
+       if (BCH_VERSION_MAJOR(new_version) >
+           BCH_VERSION_MAJOR(le16_to_cpu(c->disk_sb.sb->version)))
+               bch2_sb_field_resize(&c->disk_sb, downgrade, 0);
+
        c->disk_sb.sb->version = cpu_to_le16(new_version);
        c->disk_sb.sb->features[0] |= cpu_to_le64(BCH_SB_FEATURES_ALL);
 }